1. Field of the Invention
The present invention relates to a switching power supply circuit arranged to provide a plurality of output voltages, and more particularly it pertains to a technique for keeping the device from becoming large-sized while maintaining a desired stability of each output voltage.
2. Description of the Prior Art
It has recently been regarded as a matter of course that an electronic equipment includes a number of functional circuits or devices and two or more types (values) of driving voltages should be applied to such functional circuits or devices.
In an attempt to provide a plurality of voltages of different values, it may be considered to use a power supply apparatus including as many a power supply circuits as required to obtain such voltages. Disadvantageously, however, as the number of power supply circuits increases, the cost and size of the apparatus will also increase accordingly. Thus, it is often the case that a multi-output type switching power supply circuit is employed which is arranged to derive a plurality of output voltages of different values from a single power supply circuit.
An example of such multi-output type switching power supply circuit is shown in FIG. 3 illustrating a conventional switching power supply circuit.
To have a better understanding of the present invention, description will first be made of a conventional circuit shown in FIG. 3, wherein main current passage of a switching transistor Q1 and primary winding N5 of a transformer T3 are connected in series with each other between a high potential side input terminal 1 and ground; and a control circuit 4 is connected to the base of the switching transistor Q1. The ground serves as a reference potential point for the circuit of FIG. 3. Low potential side input terminal and each low potential side output terminal are not shown, but they are assumed to be grounded. A capacitor C1 serving as a filter is connected between the input terminal 1 and the ground. Secondary winding N6 of the transformer T3 is grounded at one end and connected at the other end to the first output terminal 2 through a diode D1; and a capacitor C2 is connected between the first output terminal 2 and the ground. Tap CT provided at a predetermined position on the secondary winding N6 is connected to second output terminal 3 through a diode D2; and a capacitor C3 is connected between the second output terminal 3 and the ground. Further, a series connection of resistors R1 and R2 for detecting output voltage is connected between the first output terminal 2 and the ground, so that voltage signal occurring at the connection point between the resistors R1 and R2 is supplied to the control circuit 4.
With the above-arranged circuit of FIG. 3, the switching transistor Q1 is repetitively turned on and off in accordance with a driving signal derived from the control circuit 4. As a result, an AC voltage is generated in the secondary winding N6 of the transformer T3. AC voltage occurring across the whole of secondary winding N6 is rectified and smoothed out by means of the diode D1 and capacitor C2 so that a first output voltage V.sub.01H occurs at the first output terminal 2. Further, an AC voltage occurring in a ground-side winding section of n62 of the secondary winding divided by the tap CT is rectified and smoothed out by means of the diode D2 and capacitor C3 so that a second output voltage V.sub.02L occurs at the second output terminal 3.
Both the first output voltage V.sub.01H and second output voltage V.sub.02L are obtained from flyback voltage occurring in the secondary winding N6; thus, the first output voltage V.sub.01H turns out to be higher than the second output voltage V.sub.02L derived from a portion of the secondary winding N6.
Voltage signal which occurs at the connection point between the resistors R1 and R2 corresponds in magnitude to the first output voltage V.sub.01H occurring at first output terminal 2. In case the first output voltage V.sub.01H is changed from a prescribed value for some reason, the control circuit 4 will operate to permit the pulse width of the driving signal to be changed in accordance with the voltage signal occurring at the connection point between the resistors R1 and R2, whereby the on-of period ratio, or on duty of the switching transistor Q1 is changed, thus resulting in flow rate per unit time of current flowing through the primary winding N5 being changed so that the amount of energy transferred from the primary side to the secondary side of the transformer T3 is changed. Thereupon, the first output voltage V.sub.01H is subjected to such an action as to cause the magnitude of the first output voltage to be returned to the original specified voltage value in accordance with the amount of transferred energy.
As described above, in the conventional circuit shown in FIG. 3, the first output voltage V.sub.01H is stabilized substantially at the specified value through control of switching operation based on the magnitude of output voltage, or so-called feedback control.
With the first output voltage V.sub.01H being stabilized at a substantially constant value, the second output voltage V.sub.02L will also be stabilized at a substantially constant value, unless there exists any external cause such as changes in an external load connected to each of the output terminals and/or changes in input voltage V.sub.IN.
The conventional switching power supply circuit such as shown in FIG. 3 is arranged such that the first output voltage V.sub.01H of a higher value is derived from the whole of the secondary winding N6. Needless to say, in an attempt to make such a switching power supply circuit, the number of turns of the secondary winding N6 of the transformer T3 is determined on the basis of the maximum one of output voltages to be obtained. Thus, it is required that the number of turns of the secondary winding N6 be increased so as to produce such a high voltage. Obviously, this will result in the transformer T3 being large-sized, which will inevitably lead to such an undesired phenomenon that the switching power supply circuit or power supply device turns out to be large-sized.
With the circuit shown in FIG. 3, the first output voltage V.sub.01H derived from the whole of the secondary winding N6 is stabilized so that the second output voltage V.sub.02L derived from part of the secondary winding N6 is also stabilized on the basis of the theory of voltage divider circuit.
In actuality, however, even if the first output voltage V.sub.01H is stabilized, the second output voltage V.sub.02L tends to be changed due to changes in various factors, especially due to great increase and decrease in output current. It may be considered that this is due to the fact that in an actual product, the electromagnetic coupling between the respective windings of the transformer constituting the power supply circuit is not perfect (the coupling coefficient is not 1.0); voltage drop resulting from current flow through electric resistance present in the respective windings and winding sections is changed with a variation in the magnitude of the current; and so forth.
As will be seen from the above discussion, the conventional switching power supply device of such a circuit arrangement as shown in FIG. 3 is disadvantageous in that in case the external load is greatly changed, it is not possible to expect a high stability of the second output voltage V.sub.02L which is not subjected to feedback control.